Peripheral prolactin levels were measured by radioimmunoassay in 9 Ile de France rams, born in autumn and reared under natural conditions of light and temperature until 3 yr old. During Dec., when the rams were 80-100 days old, a brief and transitory peak of prolactin occurred (70 ng PS6[prolactin]/ml plasma .+-. 10 ng). During each following year, the prolactin level fell in winter to approximately 20 ng/ml. In summer the prolactin level was 4 times higher than the winter level during the 1st yr and 11 times higher during the 2nd and 3rd yr. A possible role of prolactin variation throughout the breeding season is discussed.